1. Field of the Invention
The disclosure relates generally to disk drive systems, and specifically to optical disk drive circuitry for improving data detection.
2. Background of the Invention
Over the past decade, there has been a tremendous shift in portable storage from magnetic “floppy” disks to optical disks such as compact discs (CDs) and digital versatile discs (DVDs). The advantages of optical discs over their magnetic counterpart are numerous including the ability to store an enormous amount of data, making it ideal for storing large files and programs, music, movies, etc.
An important consideration in optical disk drives, and really all disk drives, is the access time, which is directly related to how fast the disk spins. That is, as the disk spins faster, the access time for reading and/or writing data is reduced. There has been much effort, in terms of time and money, in reducing the access times for optical disk drives.
Data is stored on an optical disk in the form of microscopic pits (referred to as mark for DVD) and lands (referred to as space for DVD) which separate neighboring pits. As the disk spins, pits and lands pass under a laser beam and reflect the laser beam at varying intensities. The reflected laser beam is detected by an optical pick-up unit (OPU) and, in response thereto, produces a stream of “1's” and “0's” representing a “pick-up” signal.
As optical disk drive speeds continues to increase, the components in optical disk drives such as the OPU and media become more band-limited. As a result, the OPU “pick-up” signal suffers resolution loss. Resolution may be defined as the ratio between the maximum and minimum peaks in the “pick-up” signal. For example, it has been observed in at least one commercial DVD drive that the signal resolution is 20% at the DVD 16× speed.
Optical disk drives typically employ an equalizer to boost the resolution of the “pick-up” signal. In at least one application, the minimum resolution required to detect the smallest mark is found to be about 50%, which means that the equalizer boost may have to be as big as 8 dB (=20 log10(50%/20%)). To achieve a signal resolution of about 50% at a data slicer input, it has been determined that an equalizer transfer function as expressed below in equation (1) provides good equalization.H(D)=1+2D+3D2+2D3+D4.  (1)This may be referred to as PR12321 equalization. However, by boosting the “pick-up” signal, the equalizer is also boosting the noise at the data slicer input, thereby degrading the data slicer performance.
One method for boosting the resolution of the “pick-up” signal while attempting to minimize noise includes employing a Partial Response Maximum Likelihood (PRML) technique. The PRML utilizes a Finite Impulse Response (FIR) digital filter and a Viterbi Detector (VD). The PRML technique provides reliable detection in the sense of the least mean squared error technique. One drawback of this technique, however, is the hardware complexity resulting from the speed limitation imposed by the required Add-Compare-Select (“ACS”) operation and the path memory length required for the algorithm convergence.